In recent years, to handle image information as digital information and achieve high-efficiency information transmission and accumulation in doing do, apparatuses compliant with a standard, such as MPEG (Moving Picture Experts Group) for compressing image information through orthogonal transforms such as discrete cosine transforms and motion compensation by using redundancy inherent to image information, have spread among broadcast stations that distribute information and among general households that receive information.
Particularly, MPEG2 (ISO (International Organization for Standardization)/IEC (International Electrotechnical Commission) 13818-2) is defined as a general-purpose image encoding standard, and is applicable to interlaced images and non-interlaced images, and to standard-resolution images and high-definition images. Currently, MPEG2 is used in a wide range of applications for professionals and general consumers. By using the MPEG2 compression method, a bit rate of 4 to 8 Mbps is assigned to an interlaced image having a standard resolution of 720×480 pixels, and a bit rate of 18 to 22 Mbps is assigned to an interlaced image having a high-resolution of 1920×1088 pixels, for example. Accordingly, high compression rates and excellent image quality can be realized.
MPEG2 is designed mainly for high-quality image encoding suited for broadcasting, but is not compatible with lower bit rates than that of MPEG1 or encoding methods involving higher compression rates. As mobile terminals are becoming popular, the demand for such encoding methods is expected to increase in the future, and to meet the demand, the MPEG4 encoding method has been standardized. As for image encoding methods, the ISO/IEC 14496-2 standard was approved as an international standard in December 1998.
Further, establishment of a standard called H.26L (ITU-T (International Telecommunication Union Telecommunication Standardization Sector) Q6/16 VCEG (Video Coding Expert Group)), which is originally intended for encoding images for video conferences, is currently under way. Compared with the conventional encoding methods such as MPEG2 and MPEG4, H.26L requires a larger amount of calculation in encoding and decoding, but is known to achieve a higher encoding efficiency. Also, as a part of the MPEG4 activity, “Joint Model of Enhanced-Compression Video Coding” is now being established as a standard for achieving a higher encoding efficiency by incorporating functions unsupported by H.26L into the functions based on H.26L.
On the standardization schedule, the standard was approved as an international standard under the name of H.264 and MPEG-4 Part 10 (Advanced Video Coding, hereinafter referred to as AVC) in March 2003.
However, the macroblock size of 16×16 pixels might not be optimal for a large frame such as an UHD (Ultra High Definition: 4000×2000 pixels) frame to be encoded by a next-generation encoding method.
In view of this, to achieve an even higher encoding efficiency than that achieved by AVC, an encoding method called HEVC (High Efficiency Video Coding) is now being standardized by JCTVC (Joint Collaboration Team-Video Coding), which is a joint standards organization of ITU-T and ISO/IEC (see Non-Patent Document 1, for example).
According to the HEVC encoding method, coding units (CUs) are defined as processing units like macroblocks of AVC. Unlike the macroblocks of AVC, the CUs are not fixed to the size of 16×16 pixels, and the size of the CUs is specified in the compressed image information in each sequence.
Meanwhile, to improve motion vector encoding using median predictions defined in AVC, there has been a suggestion to use not only “Spatial Predictor” but also “Temporal Predictor” and “Spatio-Temporal Predictor” as candidates for predicted motion vectors (see Non-Patent Document 2, for example).
Also, as one of motion information encoding methods, there is a suggested method called “Motion Partition Merging”, by which “Merge_Flag” and “Merge_Left_Flag” are transmitted (see Non-Patent Document 3, for example).